This invention relates to a device for producing hollow plastic articles by injection molding, the device including a nozzle having a first inlet opening for melted plastic from an injection molding machine and a second inlet opening for a pressurized gas, and an outlet opening for the gas and the melted plastic, the outlet opening being connectable to a mold via a gate.
It is known in the art to produce hollow plastic articles having walls that are shaped by a mold used with a device as set forth above. German patent DE-OS 2 461 580 describes a device in which melted plastic is ejected through a nozzle into a mold, the nozzle outlet including a central outlet for a gas. The gas and the melted plastic are transferred to the associated mold simultaneously, wherein the melted plastic surrounds the gas and forms an expanded shell which solidifies against the walls of the mold. As an example of a suitable gas, nitrogen is mentioned. In order to force the gas into and expand the melted plastic, a cylinder is used to which the gas is transferred from a suitable source. The cylinder is, via a conduit and a valve, connected to the central outlet in the nozzle. The cylinder is filled with gas, the gas is compressed by means of the piston, and the gas is then introduced into the melted plastic when the associated valve is opened.
A similar method is described in European patent EP 127 961 according to which the cylinder, which is used to force the gas into the melted plastic, also serves as a measuring or metering device for the gas in order to adapt the gas volume to the article being produced. The piston has a stroke which is variable and which can be set at a predetermined value corresponding to the gas volume which is desired. After filling the cylinder, the gas supply is cut off and the gas is compressed by moving the piston forwards. At a certain pressure the associated valve is opened and the gas expands the melted plastic within the mold cavity.
The devices described above, among other things, minimize sinkings, i.e. local deformation defects which are formed when producing thick-walled plastic articles. Moreover, a better economy is achieved compared to other devices since the consumption of plastic material is reduced. Also, a significant weight reduction of the molded article results.